The present invention relates to the fabrication of gratings in optical waveguides, specifically to the use of masks to write such gratings.
Optical fibers are now widely used for carrying light signals in optical communication systems. Gratings in the fibers are used to control those light signals. Fiber gratings can serve as filters, routers, modulators, and attenuators. Fiber gratings are also used to process the different channels in a wavelength-division multiplexed telecommunication system. Optical fiber gratings can also be used to control the output of lasers.
An important application for fiber gratings is dispersion compensation in high-speed telecommunications systems. A dispersion-compensating grating contains a variety of different grating periods. Often the grating is xe2x80x9cchirped,xe2x80x9d which means that the grating period varies with distance along the fiber grating length. To obtain precise compensation of dispersion over a wide wavelength range ( greater than 100 nm), a fiber grating needs to be physically long, up to several meters.
Several methods have been proposed for fabricating fiber gratings. The fiber can be illuminated from the side with a periodic pattern of ultraviolet light, causing a permanent change in the refractive index of the fiber""s photosensitive core. The resulting periodic index pattern forms the fiber grating.
For long-period gratings, the required period of the grating lies in the range 10 microns to hundreds of microns. Such gratings are used to direct light between different co-propagating modes of an optical fiber or waveguide.
For short-period gratings (Bragg gratings), the required period of the grating pattern is usually less than one micron. Such gratings are used to direct light between counterpropagating modes of an optical fiber or waveguide. For short-period gratings it is convenient to form the light pattern by the interference of two or more coherent light beams. However, the length of the interference pattern then limits the length of the grating, and in practice it is difficult to generate a high-quality interference pattern longer than a few centimeters.
Another technique for creating a Bragg grating is with the use of a linear phase mask, as described in U.S. Pat. Nos. 5,367,588 and 5,881,186. The linear phase mask is illuminated by a single light beam to generate a periodic diffraction pattern. Exposing the fiber to this periodic light pattern can create a Bragg grating in the fiber. UV light can be used to scan along the mask to write longer gratings. However, the length of the linear phase mask limits the length of the fiber grating. Presently a linear phase mask costs approximately $1,000 per centimeter, so a 1-meter long phase mask would be prohibitively expensive at this time. Another disadvantage of the linear phase mask technique for writing fiber gratings is that the period of the fiber grating is determined by the period of the phase mask, so many different phase masks are needed to produce many different fiber gratings.
An alternate method to make a long Bragg grating is to use several adjacent phase masks. In this case precise alignment of the phase masks is required and it is difficult to avoid stitching phase errors in the regions between two adjacent masks. Stitching errors degrade the spectrum of a fiber grating. The length of the linear mask array thus also limits the length of the grating with this technique usually to approximately 1 meter.
Another technique for making a long Bragg grating in an optical fiber uses a single, short, linear phase mask to expose multiple small sections of the fiber. The individual fiber Bragg gratings are then effectively stitched together to form a longer fiber Bragg grating. However this method invariably introduces stitching errors between adjacent gratings. Using a precision translation stage, one can move the fiber or the mask to minimize stitching errors. However, the necessary translation stage must have a precision of much better than 1 micron over a length greater than 1 meter, and such precision translation stages are complicated and expensive.
One can write a fiber grating by focusing a single laser beam to a small spot on the fiber. The fiber is then translated relative to the incident laser beam while modulating the intensity of the laser beam, for example by an amplitude mask. In this manner one can write a grating in the fiber point by point. In practice the technique is limited to gratings having a period of at least a few microns, for example, to long-period gratings. However, Bragg gratings designed for the telecommunications windows around 1500 nm and below require a grating period of  less than 0.5 micron. Therefore such a point-by-point writing method may not reliably fabricate Bragg gratings for telecommunications applications.
It would be desirable to have a method of fabricating fiber gratings with any desired length, any desired variation of period along the fiber, and without stitching errors along a grating length.
The present invention includes a mask and methods of fabricating a fiber grating using such a mask. The mask of the present invention is oriented about a central point, axis, or region, and has a pattern that varies circumferentially. The mask is preferably circular, although it could also be elliptical, spherical, cylindrical, conical, or some other configuration that is oriented about a point, region, or axis. The mask can have first and second sets of alternating sections that differ from each other in one of a number of different ways, such as different thicknesses, different materials, different transmission to actinic radiation, different reflectivity, or some combination of these differences. Alternatively, the mask can have a continuously varying pattern in a circumferential direction, such as a sinusoidal variation in reflectivity in the circumferential direction.
The present invention also includes methods for forming a grating in an optical fiber by positioning the fiber near a mask with a circumferentially varying pattern, such as a circular, elliptical, spherical, cylindrical, or conical mask, and exposing the fiber to radiation to produce a grating. The fiber can be coiled next to a mask, or it can be linearly oriented. The fiber may be moved relative to the mask by moving the fiber linearly, by rotating the mask, by rotating and translating the mask, or some combination of such movements.
The invention thus includes a mask for forming a grating in an optical fiber; an assembly for forming a grating in an optical fiber including a mask and a source of radiation; a method of fabricating an unchirped or chirped fiber grating; a method of fabricating fiber gratings having different periodicities using a single phase mask; and a method of fabricating a fiber grating having an arbitrary variation of its period along its length. The systems and methods of the present invention are versatile, inexpensive, and simple to use. Further objects and advantages will become apparent from the following detailed descriptions.